stepper as a spindle
I would guess that the max you will get out of a stepper will be like 1000 rpm ( maybe less but you'll find out ) and no torque so yo will need some ratio increase to increase the rpm.
Additionaly if you try to control it you may find out that the stepper speed will not be linear ie; rpm increase is not linear as you increase the speed due to
torque falling off and the influence of increased torque required to turn the spindle it's driving.
Just something you should be aware of.
RICH